Zonal isolation refers to the isolation of a subterranean formation or zone from other subterranean formations. The subterranean formation or zone may serve as a source of a natural resource such as oil, or water. To achieve isolation of a subterranean formation, a wellbore is typically drilled down to the subterranean formation while circulating a drilling fluid through the wellbore. After the drilling is terminated, a string of pipe, e.g., casing, is run in the wellbore. Next, primary cementing is typically performed whereby a cement slurry is placed in the annulus and permitted to set into a hard mass, thereby attaching the string of pipe to the walls of the wellbore and sealing the annulus. Subsequent secondary cementing operations such as squeeze cementing may also be performed.
The physical characteristics of a cement are generally due to the nature of the cementitious materials formed as a result of the reaction of cement with water, that is, the hydration of the cement. As hydration occurs, the cement develops various phases which impart physical characteristics, (e.g., strength), to the cement. Once the cement is mixed with water, the hydration process begins and continues for so long as cementitious material and water are present in reactive forms and quantities.
One challenge encountered in using cements for long term zonal isolation is the potential loss of structural integrity of the cement formation due to the stresses associated with the extremity of the conditions (e.g. high temperature and/or high pressure) or the cyclical stresses occurring during the life of the wellbore. Such extended time periods and/or extreme conditions may detrimentally affect various physical and/or mechanical properties of the cement such as compressive strength and/or tensile properties and may result in the loss of structural integrity. Herein, strength retrogression generally refers to the loss of structural integrity of a cement formation due to a reduction in one or more physical and/or mechanical properties (e.g., compressive strength); the conditions under which the cement may lose structural integrity (e.g., high pressure, high temperature, extended time periods) are collectively termed structural integrity compromising (SIC) conditions. For example, the reduction in mechanical properties and increase in permeability of cement at temperatures above 230° F. often due to the formation of detrimental phases (usually defined as large crystalline structures) in a cement matrix may be termed strength retrogression. Conventional methods of mitigating strength retrogression may include the addition of silica to a slurry. However, this method suffers from a variety of drawbacks. Thus, it would be desirable to develop a methodology for selecting a cement composition that can withstand SIC conditions.